1. Field of the Invention
The present invention relates to a thermal printer and, in particular, to a thermal printer in which recording paper is held by and between a thermal print head including a plurality of heating elements and a platen drum, the quantity of heat of the heating elements is selectively changed while carrying the recording paper, and an image is recorded on to the recording paper.
2. Description of the Related Art
A thermal printer has been conventionally used as a printer of a medical image diagnostic system in a medical treatment field. In other words, in the medical treatment field, there are available various kinds of medical image diagnostic appliances such as an ultrasonic image projector, an X ray CT and the like which apply ultrasonic waves, X rays or the like to a patient to detect the change of the ultrasonic waves, X rays or the like to thereby obtain an image of the affected part of the patient, and displays the image, for example, on a CRT monitor as a visual image.
In the above-mentioned medical image diagnostic appliances, it is convenient if the image of the affected part to be displayed on the monitor can be provided in the form of a hard copy. For this purpose, there have been developed various kinds of printers including a thermal printer.
Also, besides the medical use, the thermal printer has been applied to the surface printing of an ID card, a prepaid card and the like.
As the thermal printer, there are known a heat transfer type thermal printer and a heat sensitive thermal printer. Here, referring to a thermal printer of a heat transfer type with reference to FIG. 17, for example, recording paper 2 is fed one by one from a paper feed cassette 1. A platen drum 6 clamps the leading end of the recording paper by a clamper 7 and is then rotated almost half way round to a printing part. In the printing part, there are disposed a thermal print head 4 including a plurality of heating elements 3 and an ink sheet cassette 5. The thermal print head 4 and the platen drum 6 cooperate in holding the ink sheet and the recording paper 2 fed between them. In this holding condition, the heating elements 3 are driven in accordance with an image signal supplied from a control device to thereby transfer the image from the ink sheet to the recording paper 2. In order to obtain a color image, areas Y, M and C are sequentially formed in the ink sheet, and to which areas three kinds of ink, that is, yellow (Y), magenta (M) and cyan (C) referred to as subtractive primaries for printing are attached. The R, G and B signals of a color image are respectively converted to YMC signals and are then applied to the thermal head 4, so that the three-color printing can be achieved.
After completion of the printing, the platen drum 6 is rotated reversely and the printing paper is passed through a paper discharge passage 8 and is inserted into a paper discharge tray 9 with the following end of the paper entering first.
In the above-mentioned conventional thermal printer, however, due to the fact that the platen drum 6 is rotated almost half way around and is positioned at the printing part after the the recording paper 2 is clamped by the clamper 7, it takes a long time from the paper feeding to the start of the printing.
Also, in the conventional thermal printer, after completion of the printing, in order to prevent mutual interference between the recording paper 2 and the ink sheet cassette 5, or to set the following end of the recording paper at the entrance of the paper discharge passage 8, the platen drum 6 is rotated at a given angle in the same direction as in the printing time and is then rotated reversely to thereby discharge the recording paper. This results in a long printing time.
Further, according to the conventional thermal printer, an exit of the paper discharge passage 8 is opened up in a paper guide wall around the platen drum 6 and thus the paper guide wall is cut off in this portion. For this reason, when the recording paper 2 is clamped and rotated by the platen drum 6 and is passed through an opening formed in a paper feed/discharge passage 8, the recording paper 2 is caused to jump up at the edge of the opening due to its rigidness or hardness, and this jumping problem adversely effects the printing producing a slit mark.
Moreover, according to the conventional thermal printer, in order to guide the recording paper 2 around the platen drum 6, several auxiliary rollers are pressed against the platen drum 6 or a wall-shaped paper guide member is used. However, the wall-shaped paper guide member is divided into several portions because of the necessity of the paper feed/discharge opening and due to the forming limitations, and the connecting parts of the divided portions are joined with one another in a smooth manner.
For this reason, the following end of the recording paper 2 is caused to jump up at the stepped portions and clearances between the connecting portions of the paper guide member to thereby produce load variations. The load variations adversely effects the printing.
In addition, in the conventional thermal printer, the thermal print head 4 includes a plurality of heat sensitive elements arranged in a direction perpendicular to the carrying direction of the recording paper 2, and a voltage is applied based on an image signal to thereby drive the heat sensitive elements so as to be able to transfer a desired image from the ink sheet to the recording paper 2. Such a thermal print head is mounted at the two ends thereof to a arms. The thermal printer head 4 is disposed so that it oscillates pair of oscillatory arms, with respect to the platen 5 drum 6; and so that the thermal print head 4 may be brought into contact with the platen drum 6 when printing. A spring is mounted to each of the oscillatory arms and the springs press the thermal print head 4 against the platen head when the thermal print head 4 is in contact with the platen head.
However, in the conventional thermal printer, the pair of springs may sometimes have different spring forces from each other. Therefore, an even pressure cannot be applied to the platen drum 6, and an uneven pressure during printing may be produced.